Cleanliness evaluation in sputter targets using phase

ABSTRACT

An improved method and apparatus for non-destructive cleanliness evaluation in sputter targets using radio frequency waveform phase change and amplitude detection is disclosed. The apparatus acquires phase change and amplitude for a plurality of data points. The method disclosed for characterizing the sputter target material ( 52 ) employs the phase change and amplitude magnitude data for calculating cleanliness factors and generating pareto histograms.

FIELD OF THE INVENTION

[0001] This invention relates to non-destructive testing methods andapparatus for identifying types of intrinsic flaws in metallic sputtertarget materials and, more particularly, non-destructive methods andapparatus for identifying and counting of solid inclusions using radiofrequency echo waveform phase change detection.

BACKGROUND OF THE INVENTION

[0002] Cathodic sputtering is widely used for depositing thin layers orfilms of materials from sputter targets onto desired substrates such assemiconductor wafers. Basically, a cathode assembly including a sputtertarget is placed together with an anode in a chamber filled with aninert gas, preferably argon. The desired substrate is positioned in thechamber near the anode with a receiving surface oriented normal to apath between the cathode assembly and the anode. A high voltage electricfield is applied across the cathode assembly and the anode.

[0003] Electrons ejected from the cathode assembly ionize the inert gas.The electrical field then propels positively charged ions of the inertgas against a sputtering surface of the sputter target. Materialdislodged from the sputter target by the ion bombardment traverses thechamber and deposits on the receiving surface of the substrate to formthe thin layer or film.

[0004] One factor affecting the quality of the layer or film produced bya sputtering process is the “cleanliness” of the material from which thesputter target is made. The term “cleanliness” is widely used in thesemiconductor industry, among others, to characterize high purity andultra high purity materials. In common practice, “cleanliness” refers tothe degree of material internal purity. Such impurities may be present,for example, as traces of foreign elements in distributed or localizedform in the sputter target material. Cleanliness is usually measured inunits of particles per million (“ppm”) or particles per billion (“ppb”)which define a ratio between the number of contaminant atoms and thetotal number of atoms sampled.

[0005] Since the cleanliness of the material from which a sputter targetis made affects the quality of layers of films produced using thattarget, it is obviously desirable to use relatively clean materials infabricating sputter targets. This implies a need in the art fornon-destructive techniques for selecting sputter target blanks ofsuitable cleanliness to produce high quality sputter targets. Knowndestructive test methods, such as glow discharge mass spectroscopy andLECO techniques, are not suitable for this purpose.

[0006] Another factor affecting the quality of the layer or filmproduced by a sputtering process is the presence of “flaws” in thesputter target material. As used herein, the term “flaws” refers tomicroscopic volumetric defects in the sputter target material, such asinclusions, pores, cavities and micro-laminations. However, not all theflaws are “alike” in their degrading effect on sputter performance. Sometypes of flaws, for example, micro-cavities or shrinkage porosity causerelatively “mild” degrading effect on sputter performance while theothers, such as dielectric inclusions, cause a serious disturbance inthe sputter process. Therefore, there exists a corresponding need in theart for a non-destructive technique which identifies and separatelycounts different kinds of flaws which may exist in sputter targetmaterials.

[0007]FIG. 1 illustrates a prior art non-destructive ultrasonic “flaw”detection method for characterizing aluminum and aluminum alloy sputtertarget materials. The technique illustrated in FIG. 1 is similar to thatsuggested in Aluminum Pechiney PCT Application No. PCT/FR96/01959 foruse in classifying aluminum or aluminum alloy blanks suitable forfabricating sputter targets based on the size and number of internal“decohesions” detected per unit volume of the blanks.

[0008] The prior art technique of FIG. 1 employed a pulse-echo methodperformed on a test sample 10 having a planar upper surface 12 and aparallel planar lower surface 14. In accordance with this technique, afocused ultrasonic transducer 16 irradiated a sequence of positions onthe upper surface 12 of the test sample 10 with a single,short-duration, high-frequency ultrasound pulse 18 having a frequency ofat least 5 MHz, and preferably 10-50 MHz. The ultrasonic transducer 16then switched to a sensing mode and detected a series of echoes 20induced by the ultrasound pulse 18.

[0009] One factor contributing to these echoes 20 was scattering ofsonic energy from the ultrasound pulse 18 by flaws 22 in the test sample10. By comparing the amplitudes of echoes induced in the test sample 10with the amplitudes of echoes induced in reference samples (not shown)having compositions similar to that of the test sample 10 and blind,flat-bottomed holes of fixed depth and diameter, it was possible todetect and count flaws 22 in the test sample 10.

[0010] The number of flaws detected by the technique of FIG. 1 had to benormalized in order to facilitate comparison between test samples ofdifferent size and geometry. Conventionally, the number of flaws wasnormalized by volume—that is, the sputter target materials werecharacterized in units of “flaws per cubic centimeter.” The volumeassociated with the echoes 20 from each irradiation of the test sample10 was determined, in part, by estimating an effective cross-section ofthe pulse 18 in the test sample 10.

[0011] A portion of the scattered energy is attenuated by the materialmaking up the test sample 10. Furthermore, since the single flaw sizesof interest, which range from approximately 0.04 mm to 0.8 mm, are ofsame range with the wavelength of ultrasound in metals (for example, thewavelength of sound in aluminum for the frequency range of 10 MHz to 50MHz is 0.6 mm to 0.12 mm respectively), the pulse 18 has a tendency torefract around the flaws 22, which reduces the scattering intensity.

[0012] Another factor detracting from the ability of the transducer 16to detect the sonic energy scattered by the flaws 22 is the physicalnature of the substance of the flaw or more accurately a degree ofacoustic impedance mismatch at the flaw—matrix material boundary. Theimpedance mismatch directly affects the reflection and transmissioncharacteristics of ultrasound at the phase boundaries. The reflectioncoefficient of ultrasound beam at matrix-to-flaw boundary can beexpressed by the simplified expression: R=(I_(2-−I) ₁)/(I₂+I₁), where I₂is an acoustic impedance of the flaw material, and I, is an acousticimpedance of the matrix material. The simple analysis of this formulaallows us to derive several important conclusions. At first, if acousticimpedance of the flaw I₂ is less than the acoustic impedance of thematrix I₁, then the R coefficient becomes negative. The negativity ofthe R can be translated as a change in the phase of the acoustic pulsewaveform on 180°. For example, if the flaw is the gas-filled or vacuumed(shrinkage) void with the acoustic impedance equal to 0.93g/cm²-sec(×10⁶) (air) or below (vacuum), then the phase of theultrasound pulse waveform is changed on 180° at the boundary. At second,if the flaw is a gas filled or vacuumed void in the aluminum matrix withthe acoustic impedance of 17.2 g/cm²-sec(×10⁶), then the reflectioncoefficient value is close to the unity or 100% and the amplitude of thereflected signal is the only function of the relationship between flawsize and the ultrasound beam focal spot size. At third, if the flawcomprises a solid particle, for example, an alumina inclusion with theacoustic impedance of 39.6 g/cm²-sec (×10⁶), which exceeds the acousticimpedance of the aluminum matrix more than two times (17.2g/cm²sec(×10⁶)), the ultrasound waveform does not experience the phaseinversion at the flaw boundary, and for alumina inclusion the reflectioncoefficient does not exceed 39.5% of the amplitude of the impingingpulse (if the wave interference effect is not considered). In this case,the amplitude of the reflected signal is the function of two variables,firstly, the relationship between flaw size and the beam focal spotsize, and secondly, the degree of acoustic impedance mismatch atflaw-to-matrix boundary.

[0013] Therefore, the final conclusion is that the void-like flaw andalumina inclusion of same size reflect the ultrasound energy quitedifferently. In addition to the waveform phase inversion, the amplitudeof the reflected signal from the void-like flaw is at least two timeshigher than for the alumina particle inclusion. Hence, the detectabilityof alumina inclusions is generally poorer than the detectability ofvoid-like flaws, and if the phase information for reflected signal isnot extracted simultaneously with the amplitude information, the testingresults can be misleading caused by misinterpretation of the actuallarger alumina particle with the smaller void-like flaw and vice versa.

[0014] Another factor detracting from the ability of the transducer 16to detect the sonic energy scattered by the flaws 22 is the noisegenerated by scattering of the pulse 18 at the boundaries between grainshaving different textures. In fact, the texture-related noise can be sogreat for high-purity aluminum having grain sizes on the order ofseveral millimeters that small flaws within a size range ofapproximately 0.05 mm and less cannot be detected. Larger grain sizesreduce the signal-to-noise ratio for the sonic energy scattered by theflaws when compared to the noise induced by the grain boundaries.

[0015] Other factors affecting the sensitivity and resolution of thetechnique of FIG. 1 include the pulse frequency, duration and waveform;the degree of beam focus and the focal spot size; the couplingconditions (that is, the efficiency with which the sonic energy travelsfrom the transducer 16 to the test sample 10); and the data acquisitionsystem parameters.

[0016] One major drawback to the technique of FIG. 1 is an inability ofthe technique to distinguish between different sorts of flaws,particularly between void-like flaws and solid particle inclusions, suchas alumina particles. This technique, which relies only on the echoamplitude measurements, confirms only the physical existence of theflaw. Its physical nature and actual size are not properly revealed andderived only on the basis of the flaw type assumption. If the internal“decohesions” (void-like defects) are the only defects in the targetmaterial, then the technique as referred in the method (FIG. 1) is ableto detect and size defects adequately. However, in reality the internal“decohesions” as referred in the method (FIG. 1), are the fraction ofplurality of defect types which may exist in the target material. Forexample, the metallographic evaluation revealed also aluminum oxideparticles in the aluminum for sputter targets. Therefore, the techniqueas referred in the method (FIG. 1) is unable to distinguish and todifferentiate between pluralities of flaw types since the waveform phasechange information remains not revealed.

[0017] Thus, there remains a need in the art for non-destructivetechniques for characterizing sputter target materials having differentpluralities of flaw types. There also remains a need for a techniquethat separately compares the target intrinsic volumetric cleanliness forthe specific groups of flaws such as void-like flaws (cavities,microlaminations, “decohesions”) and solid inclusions.

[0018] One imaging technique implemented by Sonix, Inc. (8700 MorrisetteDr., Springfield, Va. 22152) in a FlexSCAN-C C-scanning uses a phasegating method which detects the phase inversion in the waveform at thematrix-to-flaw boundary. The technique uses a “Texas Instruments” phaseinversion algorithm (licensed to SONIX, Inc.). The technique maps theflaws on a two-dimensional sample image only if the 180° phase change isdetected. Therefore, this technique is limited to detection and mappingvoid-like defects when the impedance is changed from higher to lower atthe flaw boundary. For sputter target applications however, it isabsolutely necessary to detect and identify the aluminaparticle-inclusions, and the phase inversion technique used by theSonix. Inc. does not work in this case since the waveform does notchange its phase at the flaw boundary.

[0019] There also remains a need for a technique that separately detectsand sizes specific alumina particle-inclusions.

SUMMARY OF THE INVENTION

[0020] These needs and others are addressed by a non-destructive methodfor characterizing a sputter target material comprising the steps ofsequentially irradiating a test sample of the sputter target materialwith sonic energy at a plurality of positions on a surface of thesample; detecting echoes induced by the sonic energy; discriminatingtexture-related backscattering noise from the echoes to obtainnon-rectified radio frequency echo waveform signals; monitoringnon-rectified echo waveform signals for the 180° waveform phaseinversion, comparing the non-rectified echo waveform signals with saidat least one of each: phase inverting and phase non-inverting referencevalues, to detect void-like and particle-like flaw data pointsseparately and no-flaw data points; counting the flaw data points forthe each flaw type separately as well as all together to determine atotal flaw count C_(FT(TOTAL)); C_(FI(with phase inversion)), flaw countwithout phase inversion C_(FN(without phase inversion)), counting theflaw data points and the no-flaw data points to determine a total numberof data points C_(DP) and calculating a total cleanliness factorF_(CT)=(C_(FT)/C_(DP))×10⁶ as well as cleanliness factorsF_(CI)=(C_(FI)/C_(DP))×10⁶ and F_(C)=(C_(FN)/C_(DP))×10⁶ for each sortof flaws separately.

[0021] Unlike the prior art method described earlier, the method of thepresent invention provides a characterization of the sputter targetmaterial by separately identifying and counting void-like andparticle-like flaws. A partition of cleanliness factor for componentsassociated with different kinds of flaws tunes up the rejection criteriamore precisely by identifying and sizing the flaws of different kind.

[0022] Unlike the Sonix, Inc. method, the method of the presentinvention provides a characterization of both the waveform phaseinverting and phase non-inverting flaws. Therefore, there is a smallerrisk to miss the waveform phase non-inverting flaws which are of aprimary concern for sputter target applications.

[0023] Although the cleanliness factor technique provides a usefulcharacterization of the sputter target material, more information can beprovided by means of a histogram. More specifically, the sputter targettest method may be characterized by defining a plurality of amplitudebands for each type (waveform inverting and non-inverting) of flaws;measuring said modified amplitude signals to determine modifiedamplitude signal magnitudes; comparing said modified amplitude signalmagnitudes with said plurality of amplitude bands to form subsets ofsaid modified amplitude signals; counting said subsets of modifiedamplitude signals to determine a plurality of modified amplitude signalcounts, each modified amplitude signal count of said plurality ofamplitude signal counts corresponding to one of said amplitude bands ofsaid plurality of amplitude bands; and constructing a pareto histogram,combining individual histograms for both flaw classes, relating saidmodified signals counts to said plurality of amplitude bands. Since thehistogram does not attempt to directly map the locations of flaws alongthe surface of the sputter target material, it does not suffer from thescaling problems.

[0024] Most preferably, the test sample is compressed along onedimension, such as by rolling or forging, and then irradiated by sonicenergy propagating transversely (that is, obliquely or, better yet,normally) to that dimension. This has the additional effect offlattening and widening of certain flaws (aluminum oxide film clustersand voids) in the material. The widening of the flaws, in turn,increases the intensity of the sonic energy scattered by the flaws andreduces the likelihood that the sonic energy will refract around theflaws.

[0025] These methods for characterizing sputter target materials may beused in processes for manufacturing sputter targets. As noted earlier,the cleanliness of a sputter target and particularly cleanliness fromnon-phase inverting flaws is the primary factor determining the qualityof the layers or films produced by the target. By shaping only thosesputter target blanks having cleanliness factors or histograms meetingcertain reference criteria to form sputter targets, and rejecting blanksnot meeting those criteria, one improves the likelihood that the sputtertargets so manufactured will produce high quality layers or films.

[0026] Therefore, it is one object of the invention to providenon-destructive methods for characterizing sputter target materials.Other objects of the invention will be apparent from the followdescription, the accompanying drawings and the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

[0027]FIG. 1 is a schematic view illustrating of prior art method ofultrasonic texture analysis;

[0028]FIG. 2 is a perspective view illustrating an especially preferredtest sample, prior to compressing, used for cleanliness characterizationin accordance with the invention;

[0029]FIG. 3 is a schematic view illustrating an especially preferredmethod of ultrasonic cleanliness characterization, utilizing acompressed version of the test sample as shown in FIG. 2, in accordancewith the invention;

[0030]FIG. 4 is a schematic view of a test apparatus for carrying outthe method of FIG. 3;

[0031]FIG. 5 is a histogram characterizing a relatively “clean” Al-0.5wt % Cu material in accordance with an especially preferred form of theinvention; and

[0032]FIG. 6 is a histogram characterizing a less “clean” Al-0.5 wt % Cumaterial in accordance with the especially preferred form of theinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0033]FIG. 3 illustrates an especially preferred method for sorting offlaws and characterizing the cleanliness of sputter target material. Inaccordance with this method, a cylindrical sample 50 of the sputtertarget material (which preferably comprises metal or a metal alloy) iscompressed or worked to produce a disc-shaped test sample 52 having aplanar upper surface 54 and a planar lower surface 56 approximatelyparallel to the upper surface 54. Thereafter, a focused ultrasonictransducer 60 is positioned near the upper surface 54. The transducer 60irradiates the upper surface 54 of the test sample 52 with a single,short-duration, MHz frequency range pulse of sonic energy 62. Thetransducer 60 subsequently detects an echo 64 induced in the test sample52 by the pulse of sonic energy 62. The transducer 60 converts the echointo an electrical radio frequency signal (not shown), which isprocessed to retrieve the waveform phase and maximum amplitudeinformation.

[0034] More specifically, the sample 50, as shown in FIG. 2, first iscompressed along a dimension 70 to form the disc-shaped test sample 52as shown in FIG. 3. Preferably, the sample 50 is compressed by forgingor rolling of the sample 50, followed by diamond cutting to prepare theplanar surfaces 54 and 56. The reduction in the dimension 70 may beanywhere between 0% to 100%. The compression of the sample 50 flattensand widens certain flaws 72 to increase their surface area normal to thedimension 70.

[0035] As illustrated in FIG. 4, the test sample 52 is immersed indeionized water (not shown) in a conventional immersion tank 80. Thetransducer 60 is mounted on a mechanical X-Y scanner 82 in electricalcommunication with a controller 84 such as a PC controller. Thecontroller 84 is programmed in a conventional manner to induce themechanical X-Y scanning unit 82 to move the transducer 60 in araster-like stepwise motion across the upper surface 54 of the testsample 52.

[0036] Again, with respect to FIG. 3, the presently preferred transducer60 is sold by ULTRAN USA under the designation WS50-10P4.5. This is along focal length piezoelectric transducer having a fixed focal lengthof 114 mm (in water). At a peak frequency of approximately 9.15 MHz with8 MHz (−6 dB) bandwidth, the transducer produces a pulse of sonic energy62 having a focal zone (−6 dB) of approximately 21 mm in aluminum and afocal spot 0.8-0.9 mm in diameter.

[0037] Most preferably, the upper surface 54 of the sample 52 has awidth or diameter on the order of approximately 28 cm. Data acquisitionsteps of approximately 0.9 mm in both the “x”-direction and the“y”-direction permit the detection of 0.25 mm flat bottom holes at adetection level of −6 dB without exposure area overlap. One therebyirradiates approximately 100,000 test points on the upper surface 54.

[0038] Most preferably, the transducer 60 is oriented so that the pulseof sonic energy 62 propagates through the deionized water (not shown) inthe immersion tank 80 and strikes the test sample 52 approximatelynormal to the upper surface 54. Furthermore, the transducer 60 ispreferably spaced from the upper surface 54 such that the pulse of sonicenergy 62 is focused on a zone 86 of the test sample 52 betweenapproximately 3 mm and 24 mm below the upper surface 54. The pulse ofsonic energy 62 interacts with the sample 52 to induce echoes 64, whichthen propagate back through the deionized water (not shown) to thetransducer 60 approximately 60 μsec after the pulse of sonic energy 62is sent.

[0039] Turning back to FIG. 4, an especially preferred echo acquisitionsystem includes a low noise receiver comprising a low noise gatedpreamplifier 90; a low noise linear amplifier 92 with a set ofcalibrated attenuators, a 12-bit (2.44 mV/bit) analog-to-digitalconverter 94 and digital oscilloscope 95 connected with receiver analogoutput. When sufficient time has elapsed for the echoes to arrive at thetransducer 60, the controller 84 switches the transducer 60 from atransmitting mode to a gated electronic receiving mode. The echoes 64are received by the transducer 60 and converted into an RF electricsignal (not shown). The RF signal is amplified by the preamplifier 90and by the low noise linear amplifier 92 to produce modified amplitudesignal and displayed on the screen of oscilloscope 95 to extractwaveform phase information. The modified amplitude signal then isdigitized by the analog-to-digital converter 94 before moving on to thecontroller 84. The analog-to-digital conversion is performed so as topreserve amplitude information from the analog modified amplitudesignal.

[0040] Flaws of given nature (void-like or alumina inclusions) aredetermined by monitoring for waveform phase inversion using digitaloscilloscope 95. Flaws of given sizes are detected by comparing thedigitized modified amplitude signals obtained from the sample 52 withreference values (or calibration values) derived from tests conducted onreference samples (not shown) having compositions similar to those ofthe test sample 10 and either blind, flat-bottomed holes of fixed depthand diameter or alumina particles of given size artificially imbeddedinto reference sample material.

[0041] The especially preferred PC controller 84 controls the dataacquisition process. An especially preferred software package used inconnection with the data acquisition system is available from StructuralDiagnostics, Inc. under the designation SDI-5311 Winscan.

[0042] The PC controller 84 is also programmed to calculate the totalcleanliness factor and the cleanliness factors for the sorted flawscharacterization the material of the samples 50, 52. More precisely, itis programmed to discriminate texture-related backscattering noise andto distinguish “void-like flaw data points from the aluminaparticle-like flaw data points.” The PC controller 84 maintains a countof the flaw data points detected during the testing of a test sample 52to determine a flaw count “C_(FT)” “C_(FI),” “C_(FM)”.

[0043] The PC controller 84 also is programmed to distinguish “no-flawdata points,” that is, digitized modified amplitude signals representingamplitudes which, after comparison with the reference values, indicatethe absence of flaws.

[0044] The PC controller also determines a total number of data points“C_(DP),” that is, the sum of the flaw count CF and the number ofno-flaw data points. Although the total number of data points could bedetermined by maintaining counts of the flaw data points and the no-flawdata points, it is preferably determined by counting the total number ofpositions “C_(I)” along the upper surface 54 at which the test sample 52is irradiated by the transducer 60 and subtracting the number ofdigitized RF signals “C_(N)” which the data acquisition circuitry wasunable, due to noise or other causes, to identify as either flaw datapoints or no-flaw data points. (Alternatively, the “noise count” C_(N)may be described as the number of positions along the upper surface 54at which neither a flaw data point nor a no-flaw data point isdetected.)

[0045] Having determined the flaw counts C_(FT), C_(FI), C_(FN), and thetotal number of data points C_(DP), the PC controller is programmed tocalculate the cleanliness factor F_(C)=(C_(FT)/C_(DP))×10⁶,F_(CN)=(C_(FI)/C_(DP))×10⁶, C_(N)=(C_(FN)/C_(DP))×10⁶ to characterizethe material comprising the samples 50, 52. Unlike the prior art “flawsper cubic centimeter,” the magnitude of the cleanliness factor is notdependent on any estimate of pulse cross-sectional area. Since thecleanliness factor is normalized by the dimensionless coefficientC_(DP)×10⁻⁶ rather than by volume, it is more closely related to ppm andppb units than are units of “flaws per cubic centimeter.”

[0046] The preparation of a suitable program for determining thecleanliness factor in accordance with the invention as disclosed hereinis within the ordinary skill in the art and requires no undueexperimentation.

[0047] Another way in which to characterize the material comprising thesamples 50, 52 is by determining the size distribution of flaws in thetest sample 52. More specifically, one may characterize the cleanlinessof the sample 52 by defining amplitude bands or ranges; comparing theamplitudes represented by the digitized modified amplitude signal forcertain types of flaws (phase inverting and phase non-inverting) withthe amplitude bands to form subsets of the modified amplitude signals;counting these subsets of modified amplitude signals to determine amodified amplitude signal counts for each amplitude band and for eachtype of flaws; and constructing a pareto histogram relating the modifiedsignal counts to said plurality of amplitude bands. Since the amplitudesrepresented by the digitized modified amplitude signals for each type offlaws are related to the sizes of flaws detected in the sample 52, thehistogram provides an indication of the flaw size distribution in thesample 52.

[0048] Turning now to FIGS. 5 and 6, there may be seen pareto histogramscharacterizing two Al-0.5 wt % Cu alloy sputter target materials havingorthorhombic textures and grain sizes in the range of 0.08 mm to 0.12mm. The material of FIG. 5 was “cleaner” than that of FIG. 6; thematerial of FIG. 5 had a cleanliness factor C_(FT) of 250 and C_(FN) of100 while the material of FIG. 6 had a cleanliness factor C_(F) of 1,200and C_(FN) of 300. It is important to emphasize that for sputteringapplications to have a lower C_(FN) value is more important than to havethe lower C_(FT) value. The zone of flaw monitoring was located within agate of seven microsecond duration with a gate delay of 1 microsecond.

[0049] The abscissa 155 of the pareto histogram of FIG. 5 represents theamplitude normalized as a percentage of the echo amplitude induced in areference sample having a 0.8 mm blind, flat-bottomed hole. The ordinate157 in FIG. 5 represents the modified signal counts for each amplitude,expressed on a logarithmic scale. The echo amplitude threshold for theflaw counting was set to 12% since, as established experimentally, thetexture-related echo amplitude did not exceed 10% for all aluminumalloys tested. The abscissa 161 and ordinate 163 of the histogram ofFIG. 6 were scaled similarly.

[0050] The histograms of FIGS. 4 and 5 represent an improvement overprior art imaging techniques in that the distribution of flaw sizes maybe represented without having to represent flaw sizes relative to thesurface area of the test sample (not shown).

[0051] The preparation of a suitable program for plotting histogramssuch as those shown in FIGS. 5 and 6 in accordance with the invention asdisclosed herein is within the ordinary skill in the art and requires noundue experimentation. Either the cleanliness factor or histograms suchas those shown in FIGS. 5 and 6 may be used in a process formanufacturing sputter targets. As noted earlier, the cleanliness of asputter target is one factor determining the quality of the layers orfilms produced by the target. By shaping only those sputter targetblanks having cleanliness factors and particularly C_(FN) less thanreference cleanliness factors, or having histograms with selectedcolumns or areas less than reference values, to form sputter targets,and rejecting blanks not meeting those criteria, one improves thelikelihood that the sputter targets so manufactured will produce highquality layers or films.

[0052] While the method herein described, and the form of apparatus forcarrying this method into effect, constitute a preferred embodiment ofthis invention, it is to be understood that the invention is not limitedto this precise method and form of apparatus, and that changes may bemade in either without departing from the scope of the invention, whichis defined in the appended claims.

What is claimed is:
 1. A non-destructive method for characterizingsputter target material, comprising the steps: a) obtaining a referencevalue for waveform phase inverting flaws and a reference value forwaveform phase non-inverting flaws utilizing a reference sample; b)irradiating a test sample of said sputter target material sequentiallywith sonic energy at a plurality of positions along a surface of saidtest sample; c) detecting radio frequency echo waveforms induced by saidsonic energy; d) discriminating texture-related backscattering noisefrom said echo waveforms to obtain a radio frequency echo waveformsignal; e) monitoring said radio frequency echo waveform signal for 180°waveform phase inversion; f) comparing the radio frequency echo waveformsignal associated with each of said plurality of positions with saidwaveform phase inverting reference value and said waveform phasenon-inverting reference value and obtaining individual data pointsassociated with a void-like flaw, a particle-like flaw and no-flaw; g)counting the data points associated with waveform phase inverting flaws,waveform phase non-inverting flaws, and no-flaws to determine a flawcount associated with waveform phase inversion flaws C_(FI), a flawcount associated with waveform phase non-inversion flaws C_(FN), a totalflaw count C_(FT), and a total number of data points C_(DP); and h)calculating a total cleanliness factor F^(CT)=(C_(FT)/C_(DP))×10⁶, acleanliness factor associated with phase inversion flawsF_(CI)=(C_(FI)/C_(DP))×10⁶ and a cleanliness factor associated withphase non-inversion flaws F_(C)=(C_(FN)/C_(DP))×10⁶.
 2. Anon-destructive method for characterizing sputter target material as inclaim 1, wherein said reference sample comprises blind, flat-bottomed,holes of fixed depth and diameter.
 3. A non-destructive method forcharacterizing sputter target material as in claim 1, wherein saidreference sample comprises alumna particles of given size artificiallyimbedded.
 4. A non-destructive method for characterizing sputter targetmaterial as in claim 1, wherein said test sample is a cylindricalportion of said sputter target material.
 5. A non-destructive method forcharacterizing sputter target material as in claim 4, wherein saidcylindrical portion is formed into a disc-shaped test sample.
 6. Anon-destructive method for characterizing sputter target material as inclaim 5, wherein said disc-shaped test sample is formed by rolling saidcylindrical portion.
 7. A non-destructive method for characterizingsputter target material as in claim 5, wherein said disc-shaped testsample is formed by forging said cylindrical portion.
 8. Anon-destructive method for characterizing sputter target material as inclaim 5, wherein said disc-shaped test sample comprises first and secondplanar surfaces.
 9. A non-destructive method for characterizing sputtertarget material as in claim 8, wherein said first and second planarsurfaces are prepared by diamond cutting.
 10. A non-destructive methodfor characterizing sputter target material as in claim 1, wherein saidsonic energy is generated by a transducer.
 11. A non-destructive methodfor characterizing sputter target material as in claim 10, furthercomprising the step: immersing said test sample in deionized waterwithin an immersion tank and orienting said transducer such that saidsonic energy propagates through said deionized water striking said testsample substantially normal to an upper surface of said test sample. 12.A non-destructive method for characterizing sputter target material asin claim 10, wherein: said transducer is piezoelectric and comprises afixed focal length in water of approximately 114 mm, a peak frequency ofapproximately 9.15 MHz with approximately 8 MHz (−6 dB) bandwidth; andsaid transducer produces a pulse having a focal zone (−6 dB) ofapproximately 21 mm in aluminum and a focal spot 0.8-0.9 mm in diameter.13. A non-destructive method for characterizing sputter target materialas in claim 12, wherein: said test sample comprises an upper surfacewith a width on the order of approximately 28 cm; and obtaining saiddata points in raster-like stepwise motion in steps approximately 0.9 mmin both a x-direction and a y-direction over the entire said uppersurface.
 14. A non-destructive method for characterizing sputter targetmaterial, comprising the steps: a) defining a plurality of amplitudesignal bands for waveform phase inverting flaws; b) defining a pluralityof amplitude signal bands for waveform phase non-inverting flaws; c)irradiating a test sample of said sputter target material sequentiallywith sonic energy at a plurality of positions along a surface of saidtest sample and obtaining radio frequency echo waveforms induced by saidsonic energy; d) conditioning said radio frequency echo waveform signalto obtain modified amplitude signals; e) measuring said modifiedamplitude signals to obtain a plurality of modified amplitude signalmagnitudes; f) comparing said plurality of modified amplitude signalmagnitudes with said plurality of amplitude signal bands and formingsubsets of said magnitudes of said modified amplitude signals; g)counting said subsets of modified amplitude signal magnitudes andobtaining a plurality of modified amplitude signal magnitude counts,each modified amplitude signal magnitude count of said plurality ofamplitude signal magnitude counts corresponds to one of said amplitudesignal bands of said pluralities of amplitude signal bands; h)constructing a pareto histogram for waveform phase inverting flaws; i)constructing a pareto histogram for waveform phase non-inverting flaws;j) combining the individual histograms for waveform phase inverting andfor waveform phase non-inverting flaws; and k) relating said modifiedamplitude signal magnitude counts to said plurality of amplitude signalbands.
 15. A non-destructive method for characterizing sputter targetmaterial as in claim 14, wherein conditioning of said radio frequencyecho waveform signal is performed by amplifying and filtering said radiofrequency echo waveform signal to produce said modified amplitudesignal.
 16. A non-destructive method for characterizing sputter targetmaterial as in claim 14, wherein said modified amplitude signal isdigitized.
 17. A non-destructive method for characterizing sputtertarget material as in claim 14, wherein said cylindrical portion isformed into a disc-shaped test sample.
 18. A non-destructive method forcharacterizing sputter target material as in claim 17, wherein saiddisc-shaped test sample is formed by rolling said cylindrical portion.19. A non-destructive method for characterizing sputter target materialas in claim 17, wherein said disc-shaped test sample is formed byforging said cylindrical portion.
 20. A non-destructive method forcharacterizing sputter target material as in claim 17, wherein saiddisc-shaped test sample comprises first and second planar surfaces. 21.A non-destructive method for characterizing sputter target material asin claim 20, wherein said first and second planar surfaces are preparedby diamond cutting.
 22. A non-destructive method for characterizingsputter target material as in claim 14, wherein said sonic energy isgenerated by a transducer.
 23. A non-destructive method forcharacterizing sputter target material as in claim 22, wherein: saidtest sample is immersed in deionized water within an immersion tank andsaid transducer is oriented such that said sonic energy propagatesthrough said deionized water and strikes said test sample substantiallynormally to an upper surface of said test sample.
 24. A non-destructivemethod for characterizing sputter target material as in claim 22,wherein: said transducer is piezoelectric and comprises a fixed focallength in water of approximately 114 mm, a peak frequency ofapproximately 9.15 MHz with 8 MHz (−6 dB) bandwidth; and said transducerproduces a pulse having a focal zone (−6 dB) of approximately 21 mm inaluminum and a focal spot 0.8-0.9 mm in diameter.
 25. A non-destructivemethod for characterizing sputter target material as in claim 24,wherein: said test sample comprises an upper surface with a width on theorder of approximately 28 cm; and obtaining said data points inraster-like stepwise motion in steps approximately 0.9 mm in both ax-direction and a y-direction.
 26. An apparatus for non-destructivelycharacterizing sputter target material, comprising a transducer inelectrical communication with an echo acquisition system; saidtransducer comprises both waveform phase and waveform amplitude signals.27. An apparatus for non-destructively characterizing sputter targetmaterial as in claim 26, wherein said transducer is piezoelectric havingan in water fixed focal length of approximately 114 mm.
 28. An apparatusfor non-destructively characterizing sputter target material as in claim27, wherein: said transducer produces a pulse having a focal zone (−6dB) of approximately 21 mm in aluminum and a focal spot 0.8-0.9 mm indiameter.
 29. An apparatus for non-destructively characterizing sputtertarget material as in claim 25, wherein: said echo acquisition systemcomprises a low noise receiver connected to said transducer; and saidlow noise receiver comprises a low noise linear amplifier with a set ofcalibrated attenuators; a 12-bit, 2.44 mV/bit, analog-to-digitalconverter; and a digital oscilloscope connected to an analog output ofsaid low noise receiver.
 30. An apparatus for non-destructivelycharacterizing sputter target material as in claim 26, furthercomprising: a mechanical X-Y scanner in electrical communication with acontroller; and said transducer being mounted on said scanner by aspecifically configured mounting means.
 31. An apparatus fornon-destructively characterizing sputter target material as in claim 30,wherein: said controller is programmable; and said controller comprisesa program which controls said mechanical X-Y scanner, data acquisitionand storage of acquired data points.
 32. An apparatus fornon-destructively characterizing sputter target material as in claim 31,wherein: said program of said controller calculates a flaw countassociated with waveform phase inversion flaws C_(FI), a flaw countassociated with waveform phase non-inversion flaws C_(FN), a total flawcount C_(FT), a total number of data points C_(DP), a total cleanlinessfactor F_(CT)=(C_(FT)/C_(DP))×10⁶, a cleanliness factor associated withphase inversion flaws F_(C)=(C_(FI)/C_(DP))×10⁶ and a cleanliness factorassociated with phase non-inversion flaws F_(C)=(C_(FN)/C_(DP))×10⁶. 33.An apparatus for non-destructively characterizing sputter targetmaterial as in claim 31, wherein: said program of said controllerconstructs a pareto histogram for waveform phase inverting flaws, apareto histogram for waveform phase non-inverting flaws, combines theindividual histograms for waveform phase inverting and for waveformphase non-inverting flaws, and relates modified amplitude signalmagnitude counts to a plurality of amplitude signal bands.